In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in recent years, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate (for example, refer to Patent Document 1).
Also, Patent Document 2 discloses an ink for ink-jet printing which contains a water-insoluble colorant having a number-average molecular weight of from 10 to 100 nm under a dispersed state, a water-soluble high-molecular dispersant having a specific solubility parameter, etc., for the purposes of improving a storage stability of the ink, an unevenness of an optical density of images upon printing and a fixing property of the ink after printing.
Further, Patent Document 3 discloses a recording ink containing a solvent containing at least water and a pigment as a colorant dispersed in the solvent, in which the pigment dispersed in the solvent has an average particle size of from 0.05 to 0.1 μm, and substantially all particle sizes of the pigment are distributed in the range of from 0.01 to 0.3 μm, for the purpose of obtaining such an ink having a low viscosity and a high stability with time.
However, the above conventional inks have failed to satisfy a high rubbing resistance demanded for recent photographic images.    Patent Document 1: PCT Pamphlet WO 00/39226    Patent Document 2: JP 11-140356A    Patent Document 3: JP 07-331147A